Although containers can be made from a variety of materials, thermoplastic resins have achieved widespread acceptance over the last several decades. Molded thermoplastic containers and other articles are desirable for many applications due in part to the low cost and durability of the thermoplastic resin.
Several processes are commonly used to mold various articles from thermoplastic resins. The most common processes include injection molding, extrusion molding and blow molding. Each molding process has unique characteristics which can make the process more suitable for certain applications. Similarly, each process has disadvantages for molding certain types or shapes of articles. For example, injection molding processes are more suitable for making open containers where a thick wall is needed or where the thickness of the container is required to vary at different locations.
The primary disadvantages of injection molding processes are the increased amount of resin used to mold the container and the longer molding cycle time compared to other molding processes. These disadvantages increase the production costs of the container. However, an advantage of injection molded containers is the ability to easily control the wall thickness and the strength of the container in a manner that cannot always be obtained by other processes.
Blow molding of thermoplastic resins to make hollow articles such as containers has several advantages over other processes. For example, the blow molding processes are generally faster than some other processes, such as injection molding, thereby increasing the output capacity of the mold. The thickness of the molded resin can be made comparatively thin to reduce the cost of material in the finished article.
Blow molded containers having corrugated ribs at the top end to provide some rigidity to the container wall are common. Examples of such blow molded containers are found in U.S. Pat. No. 4,715,144 to Lee. This type of container is generally quite flexible and is not able to withstand lateral forces.
In an effort to strengthen the rim and sidewalls of the typical blow molded container, several processes have been proposed. Many of these processes utilize blow molding with moving section molds to reshape the plastic after the parison is inflated to contact the inside mold surface. The moving segments generally reshape the plastic by bending or folding the plastic while still soft. There are a number of blow molding processes which use moving section molds to fold two layers of the hot plastic over on each other and compress the hot plastic layers together. It is generally desirable to compress the plastic layers together while the plastic is still sufficiently hot so that the layers weld together. These processes typically result in a lip having greater thickness and stiffness than the remaining portions of the molded article. Examples of this type of molding process and apparatus with movable mold sections are found in U.S. Pat. No. 4,428,122 to Hammes.
Another example of a blow molding process and apparatus is disclosed in U.S. Pat. Nos. 4,972,963, 5,364,675 and 5,503,886 to Guarriello et al. The apparatus requires a number of moving mold segments which move at different and carefully controlled speeds and travel different distances to make a hollow blow molded lip. The resulting container has a tubular shaped lip encircling the open end. The tubular shaped lip is made by pinching portions of the plastic together to enclose the hollow area. Another example of a blow molded container having a hollow rim where portions of the overlapping layers are welded together is disclosed in U.S. Pat. No. 4,643,323 to Schutz.
The containers produced by the above-noted processes typically have a rigid lip providing structural integrity to the container. In some applications, such as, for example, in the nursery industry, a rigid container is desirable for lifting and stacking. A rigid lip on a nursery container has the distinct disadvantage of being sharp which can scrape and damage the bark of a plant in an adjacent container.
A lip on large nursery containers assists in lifting and carrying the nursery container. Handles formed on the inside surface of the nursery container are also advantageous in carrying large containers. Many blow molded nursery containers having inside handles do not have reinforced rims so that the container lacks structural integrity. Examples of nursery containers having a strengthening reinforced lip and having inside handles for lifting containers are disclosed in U.S. Pat. Nos. Des. 361,956 and Des. 338,636 to Moore. These containers are sufficiently rigid for lifting by the inside handle when full. However, the lips are hard and can damage the plant in an adjacent container when plants are stacked for transporting.
Accordingly, a continuing need exists in the industry for improved containers which overcome the limitations of the prior containers.